Why was the Mississippi River important to Americans who moved west after the Revolution?

It was used as the main source of irrigation for the crops that they grew.

It permitted them to move their crops to New Orleans and markets around the world.

It was a definite boundary between United States territory to the east and British territory to the west.

It enabled them to contact their biggest ally during the war, the French in New Orleans.
(D is wrong, I already took my first attempt and it was wrong.)